Active Sites For Oxygen Reduction Reaction Research Articles

Encapsulation of metal precursor within ZIFs for bimetallic N-doped carbon electrocatalyst with enhanced oxygen reduction

High-performance non-precious metal-doped carbon catalysts for oxygen reduction reactions (ORR) are viable candidates in lieu of platinum-based catalysts. It has been universally reported that active Co–N sites combined with Fe–N sites embedded in carbon matrix represent the most promising active sites for ORR process. Benefiting from the cage-encapsulated-precursor pyrolysis strategy, herein, we fabricated a Fe–N and Co–N homogeneously doped carbon framework by one step. TEM demonstrated the ultimate product had well-defined morphology with Fe (0.54 at%), Co (0.31 at%) and N (2.94 at%) uniformly distributed into the carbon skeleton. The N2 absorption-desorption isotherms indicated the MOF-derived catalyst had a high specific surface area of 647.6 m2 g−1 and inherit hierarchical porosity. Significantly, such FeCo–NC catalyst outperformed a current density (5.6 mA cm−2) at 0.70 V (vs reversible hydrogen electrode) 1.18 times higher than that of a commercial 20 wt% Pt/C (5 mA cm−2) catalyst in alkaline medium, and more positive peak potential of 0.63 V than its counterparts. Its high cycling stability and immunity towards methanol crossover in a wide range pH value showed good potential to be used as cathodes in proton exchange membrane fuel cells (PEMFCs) for long term operation. This simple synthesis strategy would to some degree leverage a cage-encapsulated-precursor for tailored utility of active sites for ORR in a porous carbon framework.

Non‐Precious‐Metal Oxygen Reduction Reaction Electrocatalysis

Non‐Precious‐Metal Oxygen Reduction Reaction Electrocatalysis

Towards High-Performance Electrocatalysts for Oxygen Reduction: Inducing Atomic-Level Reconstruction of Fe-Nx Site for Atomically Dispersed Fe/N-Doped Hierarchically Porous Carbon.

A rational and effective strategy for the synthesis of a high-performance non-precious metal electrocatalyst for oxygen reduction reaction (ORR) was developed by inducing reconstruction of Fe-Nx site on pig-bone-derived nitrogen-doped hierarchically porous carbon. The resultant Fe/N-doped carbon electrocatalyst possessed abundant atomically dispersed non-planar Fe-N4 ORR active sites, with absolute presence of active D1 (FeII -N4 ) and D3 (N-FeII -N2+2 ) sites, as well as large specific surface area and three-dimensional porous structure with hierarchical micro-/meso-/macro-pore distribution, which increased the utilization of active sites and promoted mass transport of ORR reactants. This resulted in a remarkably superior ORR activity with half-wave potential of 0.87 V (20 mV higher than Pt/C) and kinetic current density of 10.9 mA cm-2 at 0.85 V (2.3-fold of Pt/C) in alkaline electrolyte. This methodology provides a route for atomic-level design of high-performance ORR electrocatalyst.

Pyridinic-N-Doped Graphene Paper from Perforated Graphene Oxide for Efficient Oxygen Reduction.

We report a simple approach to fabricate a pyridinic-N-doped graphene film (N-pGF) without high-temperature heat treatment from perforated graphene oxide (pGO). pGO is produced by a short etching treatment with hydrogen peroxide. GO perforation predominated in a short etching time (∼1 h), inducing larger holes and defects compared to pristine GO. The pGO is advantageous to the formation of a pyridinic N-doped graphene because of strong NH3 adsorption on vacancies with oxygen functional groups during the nitrogen-doping process, and the pyridinic-N-doped graphene exhibits good electrocatalytic activity for oxygen reduction reaction (ORR). Using rotating-disk electrode measurements, we confirm that N-pGF undergoes a four-electron-transfer process during the ORR in alkaline and acidic media by possessing sufficient diffusion pathways and readily available ORR active sites for efficient mass transport. A comparison between Pt/N-pGF and commercial Pt/C shows that Pt/N-pGF has superior performance, based on its more positive onset potential and higher limiting diffusion current at −0.5 V.

Catalyst Design for Oxygen Reduction Reaction Using Pyridinic Nitrogen-Doped Carbon Materials

Nitrogen-doped graphitic carbons as non-Pt catalysts for oxygen reduction reaction (ORR) have been paid much attention in recent years because commercialization of fuel cells requires less expensive and abundant catalytic materials. Identification of the active site of nitrogen-doped carbon materials for ORR is thus urgently required, but still under debate. Currently, the debate focuses on whether the active site is created by pyridinic N (N bonded to two carbon atoms) or graphitic N (N bonded to three carbon atoms, also called substituted N or quaternary N). To determine the active site conclusively, we prepared model catalysts of highly oriented pyrolytic graphite (HOPG) with pyridinic N (pyri-HOPG) or graphitic N (grap-HOPG). The active sites and adsorption properties were examined by ORR and post-ORR X-ray photoelectron spectroscopy (XPS). We have thus determined the active nitrogen species in carbon [1].The graphitic-N doping was performed by mild bombardment with a nitrogen ion beam. To prepare the pyri-HOPG samples, edge patterning was first performed by bombarding the sample with an Ar+ ion beam through a slit-patterned Ni mask as shown in Fig.1 A-D. The edged HOPG samples were then exposed to NH3 at 973 K. The catalytic performances of the model catalysts were measured by cyclic voltammetry (CV) in acidic electrolyte (0.1 M H2SO4). It was found that the pyri-HOPG model catalyst shows high activity at high voltages, compared to the very low ORR activities of the N-free model catalysts (Fig.1 E, F). The pyri-HOPG sample with lower N concentration (N: 0.60 at%) is much more active than the grap-HOPG sample with higher N concentration (N: 0.73 at%). Since the pyri-HOPG sample is nearly free of graphitic N, the ORR results indicate that it is pyridinic N rather than the graphitic N that reduces the ORR overpotential and creates the active site. It is thus concluded that the ORR active sites in nitrogen-doped carbon materials are created by pyridinic N. Based on the model catalyst study, we then try to prepare catalytically active carbon surfaces covered with pyridinic nitrogen-containing aromatic molecules. That is, bottom-up preparation of catalysts using HOPG electrode covered with pyridinic nitrogen-containing aromatic molecules (dibenz[a,c] acridine (DA) molecule). The DA molecules were deposited on HOPG with different coverage by simply dropping solutions of the DA molecules at room temperature. Scanning tunneling microscopy (STM) measurements revealed that a well-ordered two-dimensional structure of DA monolayer is formed on HOPG surfaces with high densities via π-π interaction, rather than aggregates to form three-dimensional clusters. The nitrogen concentration of the DA-covered HOPG surfaces was estimated to be 0.5~1.5 at.% by XPS. The DA-covered HOPG model catalysts reveled activities of ORR. The specific activity per pyridinic nitrogen atom was estimated to be 0.08 (e sec-1 pyriN-1) at 0.3 eV, which is comparable to that for pyridinic nitrogen incorporated graphene sheets (0.07 ~ 0.14 (e sec-1 pyriN-1))[1]. The current densities at 0.1, 0.2, and 0.3 V vs RHE were in proportional to the surface coverage of DA molecules, indicating that the ORR active site was created by DA molecule adsorbed on HOPG. The present studies clearly show that fixing nitrogen-containing aromatic molecules on graphitic carbon materials is one of the promising approaches to prepare active ORR carbon catalysts. Acknowledgement: This study is supported by JST PRESTO and NEDO project. REFERENCES [1] D. Guo, R. Shibuyaa, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Science, 351 (2016) 361. Figure 1

Operando Determination of Oxygen Reduction Reaction Kinetics on PGM-Free Electrocatalysts in a PEFC

While several decades of research have been dedicated to improving the understanding of the electrochemical kinetics on Pt-based oxygen reduction reaction (ORR) electrocatalysts,1–3 similar experimental and modeling efforts for platinum group metal-free (PGM-free) electrocatalysts have been less prevalent.4 Limitations in PGM-free electrocatalyst/electrode stability5 and active-site quantification6 are perhaps the biggest barriers against extracting electrochemical kinetic data from operando studies at the MEA level. However, recent efforts in PGM-free catalyst synthesis at Los Alamos National Laboratory (LANL),7 have yielded a robust PGM-free electrocatalyst capable of producing repeatable polarization curves over several hours of testing. This electrocatalyst is derived from a pyrolyzed Fe-Zn metal organic framework containing “atomically dispersed” N-coordinated Fe groups, (AD)Fe-N-C, which may act as the ORR active sites.7,8 Here, we present a systematic operando electrochemical kinetic examination of the LANL (AD)Fe-N-C catalyst in a membrane-electrode assembly in the polymer electrolyte fuel cell (PEFC). These experiments were performed using a 5 cm2 differential cell (i.e., high oxygen flow rates and thus high stoichiometry).3 Based on the assessment of the cathode proton resistance, oxygen reaction order and ORR activation energy, we propose the inclusion of a potential-dependent descriptor for the availability of electrochemical active sites, as shown in Eq. 1 below: i + is = i0 (1 – θ) exp(αnFη/RT) [1] Here, i, is and i0 are, respectively, the measured, shorting, and ORR exchange current densities, (1 – θ) is the fraction of active sites available for ORR, and ηc is the ORR kinetic overpontential.9 As seen in Fig. 1a, the availability of active sites has a sigmoidal dependence on the cathode potential, with the largest increase in site availability occurring between 0.7 and 0.8 V. This finding will be discussed in the context of the potential window, over which the Fe3+/Fe2+ transition occurs.4,10–1 2 When the potential dependence of the PGM-free active site availability is incorporated into the electrochemical model, it results in an excellent fit of the experimental data, depicted in Fig. 1b. The results from this experimental/model approach not only offer guidance for means to improve the performance of (AD)Fe-N-C catalysts, but also increase our understanding of the kinetic limitations of other PGM-free ORR electrocatalysts systems. Figure 1. a) Potential-dependent availability of ORR active sites. b) Fitting of the modeling results with experimental data from polarization curves taken at different O2 partial pressures for PEMFC fabricated with the LANL PGM-free catalysts at the cathode (80oC, 100% RH, 300/300 sccm H2/O2, 3.5 mgcat/cm2 elec).

(Keynote) The Progress and Challenges in Oxygen Reduction Electrocatalysis without Precious Metals

The focus of this presentation will be the relatively new class of non-precious metal (or platinum group metal-free, PGM-free) electrocatalysts for oxygen reduction reaction (ORR). This particular class of materials has been the subject of a growing interest worldwide since the late 1990s, as one of two main approaches to address the catalyst-cost challenge of polymer electrolyte fuel cells (PEFCs) and some other electrochemical systems that heavily depend on precious metals, typically platinum and its alloys, for oxygen reduction. (With the other approach relying on various ways of Pt thrifting, for example, alloying, core-shell nanoparticle catalyst development, and thin-layer catalyst designs.) The most successful methods of synthesizing PGM-free ORR catalysts typically involve the heat treatment of precursors of transition metals, nitrogen and carbon. These methods have become more sophisticated over the years, yielding catalysts with oxygen reduction activity approaching that of precious metal-based formulations already implemented in the first commercially available fuel cell cars. The most impressive progress in the field appears to have been achieved predominantly via the improvements to the ORR active-site density and catalyst-layer porosity, especially microporosity, using pore-forming agents, sacrificial templates and by other approaches. All the recent progress in PGM-free electrocatalysis notwithstanding, the growing conviction in the electrocatalysis community in the last few years has been that further development of such catalysts hinges on the understanding of the source of the ORR activity in materials without any precious metals and, possibly, without obvious two adjacent active sites for promoting the dissociative mechanism of oxygen reduction. Such understanding is also needed to reveal the reasons for the relatively fast performance loss in fuel cells and also in the electrochemical cells, as well as for the development of remediation strategies, which, together with the activity enhancements, are required to overcome the performance gap between non-precious materials and the incumbent Pt-based cathode catalysts. In this presentation, we will concentrate on both the unquestionable progress in the PGM-free electrocatalysis, accomplished over a relatively short period of time, and significant challenges still facing the field. In the context of challenges, we will summarize efforts specifically targeting better understanding of the sources of PGM-free catalysts activity, identification and quantification of the ORR active sites, and the reaction mechanism, as important steps towards a rational design of catalysts in the future. The results from both theoretical and experimental studies will be presented. Of a number of both in situ and ex situ techniques used for gaining a better insight into the origins of ORR activity in PGM-free catalysts, we will concentrate on the microscopic and X-ray absorption spectroscopic methods, as well as on the implementation of molecular dioxygen analogues as probes for the ORR active sites on the PGM-free catalyst surface, which promises to make otherwise bulk techniques, surface-specific. The characterization part of the presentation will include research performed in close collaboration with partners in the DOE Electrocatalysis Consortium (ElectroCat).

Electrodeposited Transition Metal Oxides As Separate Electrodes for Rechargeable Zinc-Air Batteries

The rapid growth of renewable energy production requires an economical and efficient way to store and deliver the electricity. Electrically rechargeable Zinc-air batteries have gained revived interest among the various technologies available with their high theoretical energy density and low cost1. However, the large-scale industrial deployment of zinc-air batteries has been hampered by several problems, i.e., low round-trip energy efficiency and reduced cycling stability, both of which can be primarily attributed to the degradation of the air electrodes. Unlike conventional batteries like Li-ion, the charging and discharging processes for zinc-air batteries have different requirements for the electrodes. The discharge process is driven by the oxygen reduction reaction (ORR) and requires an air electrode that is not flooded by the electrolyte. The charging process (oxygen evolution reaction, OER), on the other hand, is more favored when the electrode is submerged in the electrolyte. In addition, the ORR active sites at the electrode can be damaged by the oxidation potential of the OER process2. Therefore, a design with physically decoupled electrodes for discharge and charge can avoid these adverse effects. This approach also allows for more flexibility to optimize ORR and OER electrocatalysts individually. In this study, ORR and OER active catalysts based on transition metal oxides are electrodeposited on different current collectors. The ORR catalyst is based on manganese oxides, while the OER catalyst is a cobalt-iron solid solution oxide. Scanning electron microscopy (SEM) reveals that both catalysts are nanostructured with high surface areas for electrochemical reactions. Electrochemical tests show that the both catalysts have comparable or even better activity than their commercial Pt-Ru catalyst counterparts. The durability of the manganese oxide catalyst is significantly enhanced by using it exclusively for ORR instead of as an ORR-OER bifunctional catalyst. The catalysts were assembled into a zinc-air battery as decoupled electrodes for discharge and charge tests. Preliminary cell testing shows that the discharge-charge efficiency was around 59% at 10 mA/cm2 current density. Both electrodes demonstrate excellent stability after 50 hours of battery testing.

Size-Tunable Atomic Iron Catalysts Derived from Metal-Organic Framework for Oxygen Reduction in Acid Media

The high cost platinum (Pt) catalyst used for the electrocatalysis of oxygen reduction reaction (ORR) is one of the bottlenecks for the widespread deployment of polymer membrane electrolyte fuel cells (PEMFCs). Fe-N-C catalysts, produced by the pyrolysis of iron, nitrogen and carbon precursors together, have been found as one of the promising alternatives to replace Pt with abundant elements and low cost.1-3 The Fe-N coordination structure bonded with carbon, generated during high-temperature pyrolysis, has been identified as the active site for the high ORR activity of Fe-N-C catalysts. However, it remains a grand challenge to prepare Fe-N-C catalysts with abundant Fe-N active sites to achieve high ORR activity, since undesired inactive species (e.g., Fe/Fe3C) are easily obtained during high-temperature pyrolysis due to the improper design and the poor chemistry control in the integration of all precursors, lowering the active sites density of catalysts. We have developed an approach to synthesize Fe-N-C catalysts with exclusively atomic Fe-N active sites instead of inactive Fe agglomeration by using well-defined Fe-containing metal-organic frameworks (MOFs) precursors.4 The morphology of MOF precursors can be directly transferred to Fe-N-C catalysts with the retained and homogenous morphology after pyrolysis. Such well-defined precursors and resulting homogenous catalysts allow us to precisely control and tune the composition and morphology of final Fe-N-C catalysts to understand how the property change of catalysts impact their ORR activity of catalysts. In this presentation, we will discuss the effect of particle size of catalysts on their ORR activity and the critical role of pyrolysis temperature on the formation of Fe-N active sites. The Fe-N-C catalysts with size from 20 nm to 1000 nm are prepared by adjusting the size of MOF crystals in the precursor synthesis. Similar to Pt nanoparticles, the unique size control of the Fe-N-C catalysts enables us to increase the accessible number of Fe-N active sites for ORR. The 50 nm catalyst shows the best ORR activity with a half-wave potential of 0.85 V vs. RHE, only leaving 30 mV gap with Pt/C (60 µgPt/cm2) in 0.5 M H2SO4 along with the excellent stability. When the particle size of catalyst is reduced to 20 nm, significant agglomeration of particles are found in the catalyst, resulting in ORR activity decrease of the catalyst. Using our homogenous model catalysts, the formation of active sites during pyrolysis is investigated by correlated the measured-ORR activity with the bonds change of precursors at various pyrolysis temperature. 800 oC is found to be the critical temperature to form the Fe-N active sites with notable ORR activity, which is related to the generation of new Fe species likely bonded with pyridinic N in the carbon structure. These high performance Fe-N-C catalysts exhibit a promising potential to replace Pt for ORR in future PEMFCs. (1) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443. (2) Zhang, H.; Osgood, H.; Xie, X.; Shao, Y.; Wu, G., Nano Energy, 2017, 31, 331-350. (3) Wu, G.; Santandreu, A.; Kellogg, W.; Gupta, S.; Ogoke, O.; Zhang, H.; Wang, H.-L.; Dai, L., Nano Energy, 2016, 29, 83–110. (4) Zhang, H.; Hwang, S.; Wang, M.; Feng, Z.; Karakalos, S.; Luo, L.; Qiao, Z.; Xie, X.; Wang, C.; Su, D.; Shao, Y.; Wu, G. Journal of the American Chemical Society, 2017, 139, 14143–14149.

(Invited) Active Carbon Supports for Pt Cathode Catalysts in PEM Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) have been widely considered one of environmentally friendly and highly efficient electric power sources for a wide variety of power applications ranging from portable and stationary power supplies to transportation. However, some of the most critical barriers to their commercialization are the intrinsic high cost and insufficient long-term performance durability. Among others, the spherical carbon black particle support used for Pt catalyst in MEAs have a number of intrinsic structural disadvantage. It is partially responsible for current insufficient performance and durability of MEAs. They include (1) inefficient O2 transport due to significant tortuosity in carbon black-based cathode, (2) non-uniform distribution of ionomer due to significant microporosity of carbon blacks, (3) catastrophic corrosion during start-up/shut-down events due to low corrosion resistance, (4) low electrical conductivity, and (5) weak binding sites with Pt nanoparticles. We has developed a new type of high-surface areas (up to 850 m2/g) and large size nitrogen-diped graphene tubes (NGT) in a highly controlled manner and at high yields. The diameters of these graphene tubes can be tuned from 50 to 500 nm, which is much larger than conventiona carbon nanotubes (20 nm). It should be noted that the graphene tube is low cost using inexpensive dicyandiamide as precursors ($1-2/kg) and the synthesis method is easily scaled up. Apart from the obvious advantage of high electronic conductivity and enhanced corrosion resistance, the large tube size provided better Pt nanoparticle dispersion and more facil mass transfer including O2 and H2O. Importantly, the NGT exhibited oxygen reduction reaction (ORR) activity in acidic media comparable with state of the art PGM-free catalsyts. Thus, the novel ORR active nitrogen-doped graphene tubes are able to not only offer remarkable support effects by geometrically and electronically modifying the deposited Pt nanoparticles, but also provide a large amount of complementary nonprecious ORR active sites, thereby significantly improving the overall activity and durability of the Pt catalysts. Thus, relative to conventional carbon blacks, the highly graphitized and active N-GT is an ideal support for low-PGM catalysts for high-performance MEAs with enhanced mass transport and durability along with ideal interfaces with ionomer.

Sustainable Synthesis of Co@NC Core Shell Nanostructures from Metal Organic Frameworks via Mechanochemical Coordination Self-Assembly: An Efficient Electrocatalyst for Oxygen Reduction Reaction.

Herein, a new type of cobalt encapsulated nitrogen-doped carbon (Co@NC) nanostructure employing Znx Co1-x (C3 H4 N2 ) metal-organic framework (MOF) as precursor is developed, by a simple, ecofriendly, solvent-free approach that utilizes a mechanochemical coordination self-assembly strategy. Possible evolution of Znx Co1-x (C3 H4 N2 ) MOF structures and their conversion to Co@NC nanostructures is established from an X-ray diffraction technique and transmission electron microscopy analysis, which reveal that MOF-derived Co@NC core-shell nanostructures are well ordered and highly crystalline in nature. Co@NC-MOF core-shell nanostructures show excellent catalytic activity for the oxygen reduction reaction (ORR), with onset potential of 0.97 V and half-wave potential of 0.88 V versus relative hydrogen electrode in alkaline electrolyte, and excellent durability with zero degradation after 5000 potential cycles; whereas under similar experimental conditions, the commonly utilized Pt/C electrocatalyst degrades. The Co@NC-MOF electrocatalyst also shows excellent tolerance to methanol, unlike the Pt/C electrocatalyst. X-ray photoelectron spectroscopy (XPS) analysis shows the presence of ORR active pyridinic-N and graphitic-N species, along with CoNx Cy and CoNx ORR active (M-N-C) sites. Enhanced electron transfer kinetics from nitrogen-doped carbon shell to core Co nanoparticles, the existence of M-N-C active sites, and protective NC shells are responsible for high ORR activity and durability of the Co@NC-MOF electrocatalyst.

Cobalt Sulfide Nanowires Core Encapsulated by a N, S Codoped Graphitic Carbon Shell for Efficient Oxygen Reduction Reaction.

Exploration of economical electrocatalysts for highly efficient and stable oxygen reduction reaction (ORR) is believed to be essential for diverse future renewable energy applications. Herein, cobalt sulfide nanowire core encapsulated in a N, S codoped graphitic carbon shell (CoS NWs@NSC) is successfully fabricated via the calcination of polydopamine-coated Co(CO3 )0.5 (OH)0.11 H2 O NWs with sulfur powder under argon atmosphere. The uniform encapsulation of CoS core by N, S codoped graphitic carbon shell favors the interaction of the core-shell structure for generating stable and numerous ORR active sites homogeneously dispersed throughout the materials. Meanwhile, the wire-like CoS NWs@NSC stacks to form 3D mesoporous conductive networks, which improves the mass and charge transport capability of catalyst. Accordingly, the resultant CoS NWs@NSC electrocatalysts possess excellent ORR activity through the four-electron pathway with superior stability and methanol tolerance over the Pt/C in 0.1 m KOH. This strategy can offer inspiration for designing and fabricating novel core-shell-structured nanomaterials with active sites derived from uniform morphology as potential electrocatalysts for various vital renewable energy devices.

Toward Pt-Free Anion-Exchange Membrane Fuel Cells: Fe–Sn Carbon Nitride–Graphene Core–Shell Electrocatalysts for the Oxygen Reduction Reaction

We report on the development of two new Pt-free electrocatalysts (ECs) for the oxygen reduction reaction (ORR) based on graphene nanoplatelets (GNPs). We designed the ECs with a core-shell morphology, where a GNP core support is covered by a carbon nitride (CN) shell. The proposed ECs present ORR active sites that are not associated to nanoparticles of metal/alloy/oxide, but are instead based on Fe and Sn sub-nanometric clusters bound in coordination nests formed by carbon and nitrogen ligands of the CN shell. The performance and reaction mechanism of the ECs in the ORR are evaluated in an alkaline medium by cyclic voltammetry with the thin-film rotating ring-disk approach and confirmed by measurements on gas-diffusion electrodes. The proposed GNP-supported ECs present an ORR overpotential of only ca. 70 mV higher with respect to a conventional Pt/C reference EC including a XC-72R carbon black support. These results make the reported ECs very promising for application in anion-exchange membrane fuel cells. Moreover, our methodology provides an example of a general synthesis protocol for the development of new Pt-free ECs for the ORR having ample room for further performance improvement beyond the state of the art.

Refining cocoon to prepare (N, S, and Fe) ternary-doped porous carbon aerogel as efficient catalyst for the oxygen reduction reaction in alkaline medium

Various advanced sulfur doped Fe-N-C non-noble metal catalysts of oxygen reduction reaction (ORR) have been recently designed and reported with excellent catalytic activity. Herein, we refined cocoon with several steps to form silk fibroin solution, treated with iron salt to prepare an easy available, heteroatom (N, S, and Fe) ternary-doped, porous carbon aerogel (HDCA). Heteroatom existed in organic compounds in silk fibroin endow active site for ORR of the resultant carbon frameworks. Moreover, the amino acids presented in silk fibroin acted as ligands, functioning with Fe ions to form FeNx coordination compounds, which also served as active sites towards ORR. The synthesized HDCA electrocatalysts, especially HDCA-800 (obtained at 800 °C) displayed excellent catalytic activity with onsets, half-wave potential of 0.94 V, 0.79 V and higher limited current density of 3.80 mA cm−2 through a near four-electron reduction pathway with an average electron transferred number of 3.86, making them promising alternatives for state-of-the-art ORR electrocatalysts in fuel cell field. The porous structure with synergistic effect of N and S heteroatom doping has been proposed to play a key role in facilitating the desired ORR reaction.

Synthesis of pyridinic-N doped carbon nanofibers and its electro-catalytic activity for oxygen reduction reaction

As catalytic active sites of nitrogen (N)-doped carbon nanomaterials (CNMs) for oxygen reduction reaction (ORR) are always under debate, synthesis of high purity pyridinic-N dominated CNMs is an effective approach to study the relation between pyridinic-N and active sites, as well as catalytic mechanism for ORR. Herein, the carbon nanofibers (CNFs) etched by hydrofluoric (HF) acid can be used to synthesize pure pyridinic-N, suggesting that the exposed edge-patterned surface facilitates to synthesize pure pyridinic-N. Moreover, it reveals that the carbon atoms adjacent to pyridinic-N could be the catalytic active sites for ORR.

Yttria stabilized and surface activated platinum (PtxYOy) nanoparticles through rapid microwave assisted synthesis for oxygen reduction reaction

The enhancement of platinum (Pt) based catalysts for the oxygen reduction reaction (ORR) by addition of rare earth metals represents a promising strategy to achieve high activity yet low content of the precious metal and concurrently addresses stability issues experienced by traditional late transition metal doping. Improvement in Pt utilization is essential for vehicular applications where material cost and abundancy is a great concern. Here we report a fast and efficient production route of yttria-stabilized platinum nanoparticles (PtxYOy) using a conventional household microwave oven. ORR performance showed a significant improvement and an optimum activity at a 3:1 Pt:Y ratio outperforming that of commercial Pt-Vulcan with a doubled specific activity. Incorporation of Y is evidenced by extended X-ray absorption fine structure and energy dispersive X-ray analysis, while significant amounts of integrated Y2O3 species are detected by X-ray photoelectron spectroscopy. Density functional theory calculations suggest surface migration and oxidation of Y, forming stable superficial yttrium oxide species with low negative enthalpies of formation. The robustness of PtxYOy is shown experimentally and through theoretical arguments demonstrating that surface yttria acts as an stabilizing agent and promoter of highly active ORR sites on the remaining Pt surface, surpassing even the Pt3Y alloy configuration.

Cobalt nitride nanoparticle-modified nitrogen-doped graphene aerogel used as an efficient catalyst for oxygen reduction reaction in acidic medium

A promising cobalt nitride nanoparticle-modified nitrogen-doped graphene aerogel (CoN–NGA) has been prepared by chemical reduction and thermal treatment for oxygen reduction reaction (ORR). X-ray diffractometry result reveals the presence of cobalt nitride (CoN) nanoparticles on aerogel. X-ray photoelectron spectroscopy analyses reveal that nitrogen atoms have been doped into framework of graphene sheet and nitrogen-doped graphene sheets have been modified with cobalt nitride nanoparticles through CoN–C moieties. Analyses of scanning electron microscopy and Brunauer–Emmett–Teller show that CoN–NGA catalyst displays a various and interconnected pore structure. Because of a huge synergistic effect of the nitrogen-doped graphene, the three-dimensional porous structure and cobalt nitride nanoparticles, the prepared CoN–NGA catalyst displays comparable electrocatalytic activity and the similar onset potential as commercial Pt/C (20% loading). CoN–NGA catalyst exhibits superior stability and excellent methanol tolerance in acidic solution. Nitrogen-doped graphene has been provided positive potential and excellent methanol tolerance through functionalizing with cobalt nitride nanoparticles, because there are strong interactions between N-doped graphene and CoN nanoparticles through CoN–C moieties. It is found that CoN–C moieties are actual ORR active sites rather than CoN nanoparticles themselves. The possible mechanism of ORR on CoN–C moieties has been involved. This study demonstrates a feasible strategy to design non-precious metal nitride-modified heteroatom-doped graphene electrocatalyst for ORR in acidic electrolyte.

How heteroatoms (Ge, N, P) improve the electrocatalytic performance of graphene: theory and experiment

The oxygen reduction reactions (ORR) play a crucial role in the electrochemical energy storage devices, such as fuel cell, metal-air batteries [1], [2], [3]. However, their larger scale applications are hindered by the sluggish slow kinetics of the ORR. Up to now, platinum (Pt)-based catalysts are still known as the best ORR electrocatalysts owing to the relatively low overpotential, high catalytic activity and current density [4]. Nevertheless, the scarcity and high cost of Pt-based materials [5] limit their wider application. Therefore, the search and development of highly efficient and cost-effective catalysts for ORR is highly desirable. Recently, an enormous amount of research effort goes into the synthesis of carbon-based electrocatalysts doped by heteroatoms (e.g., N) to replace Pt-based catalysts. The introduction of N atoms break the sp2 hybridization of carbon, inducing a lone pair of electrons, thus altering the charge and spin density of adjacent carbon atoms. Therefore, the modified carbon materials generate active sites for ORR.

Graphene-templated synthesis of sandwich-like porous carbon nanosheets for efficient oxygen reduction reaction in both alkaline and acidic media

Developing low-cost, high-performance electrocatalysts for the oxygen reduction reaction (ORR) is crucial for implementation of fuel cells and metal-air batteries into practical applications. Graphene-based catalysts have been extensively investigated for ORR in alkaline electrolytes. However, their performance in acidic electrolytes still requires further improvement compared to the Pt/C catalyst. Here we report a self-templating approach to prepare graphene-based sandwich-like porous carbon nanosheets for efficient ORR in both alkaline and acidic electrolytes. Graphene oxides were first used to adsorb m -phenylenediamine molecules which can form a nitrogen-rich polymer network after oxidative polymerization. Then iron (Fe) salt was introduced into the polymer network and transformed into ORR active Fe–N–C sites along with Fe, FeS, and FeN0.05 nanoparticles after pyrolysis, generating ORR active sandwich-like carbon nanosheets. Due to the presence of multiple ORR active sites. The as-obtained catalyst exhibited prominent ORR activity with a half-wave potential ~30 mV more positive than Pt/C in 0.1 mol L−1 KOH, while the half-wave potential of the catalyst was only ~40 mV lower than that of commercial Pt/C in 0.1 mol L−1 HClO4. The unique planar sandwich-like structure could expose abundant active sites for ORR. Meanwhile, the graphene layer and porous structure could simultaneously enhance electrical conductivity and facilitate mass transport. The prominent electrocatalytic activity and durability in both alkaline and acidic electrolytes indicate that these carbon nanosheets hold great potential as alternatives to precious metal-based catalysts, as demonstrated in zinc-air batteries and proton exchange membrane fuel cells.

Creation of oxygen reduction reaction active sites on titanium oxynitride without increasing the nitrogen doping level

The oxygen reduction reaction (ORR) active sites on titanium oxynitride (TiOxNy) have been investigated using catalysts with various nitrogen doping levels, leaving some margins to determine their origin. Herein, low-temperature annealing enhanced the ORR activity of carbon-support-free TiOxNy with a constant nitrogen doping level, indicating that oxygen vacancies produced the sites.